This laboratory study examined human stair ascending capacity and constraining factors including legs’ local muscle fatigue (LMF) and cardiorespiratory capacity. Twenty-five healthy volunteers, mean age 35.3 years, with maximal oxygen uptake (VO2max) of 46.7 mL·min-1·kg-1, and maximal heart rate of 190 bpm, ascended on a stair machine at 60% and 75% (3 min each), and 90% of VO2max (5 min or until exhaustion). The VO2, HR and electromyography (EMG) of the leg muscles were measured. The average VO2highest reached 43.9mL·min-1·kg-1,and HRhighest peaked at 185 bpm at 90%of VO2max step rate (SR). EMG amplitudes significantly increased at all three levels, p<0.05, and median frequencies decreased mostly at 90% of VO2maxSR evidencing leg LMF. Muscle activity interpretation squares were developed and effectively used to observe changes over time, confirming LMF. The combined effects of LMF and cardiorespiratory constraints reduced ascending tolerance and constrained the duration to 4.32 minutes.